Electroacoustic system and means



Dec. 26, 1950 s, JOHNSON 2,535,681

ELECTROACOUSTIC SYSTEM AND MEANS Original Filed Aug. 7, 1946 5 Sheets-Sheet 1 NW/ FIG. 2 M V R/ R2 FIG. 11 10 N M L ACOLGT/CALLY ML "HYBRID LAMP "T'" 2w" gm? NW8 PIE m R2 lvwz 212 FIG. 3

ACOUSTICALLY COM/U634 TE PA Th RECEIVER PHONE 0 MOUTH I INPUT l l I T n F/G. 7

orrmmuso ACOUSTIC R7 00 av PHrs/cAL CHARACTERISTICS CHAMBER MICRO Y I Z 20 HORN sg fim 0 EARS i g X9 E I I RES/S A c I I PZRSONS M/c/POPHONE E R5 z AZDUTH OUTPUT F IG. 8

R8 [L8 T6 r lNl/ENTOR Kl S JOHNSQ/V GPM I 5 ATTORNEY Dec. 26, 1950 K. s. JOHNSON 2,535,631

ELECTROACOUSTIC SYSTEM AND MEANS Original Filed Aug. 7, 1946 5 Sheets-Sheet 2 FIG. 9 FIG. /3

r L H4 INVENTOR y KS. JOHNSON Dec. 26, 1950 s, JOHNASON 2,535,681

ELECTROACOUSTIC SYSTEM AND MEANS Original Filed Aug. 7, 1946 s Shets-Sheet 3 FIG. /8

.ewcwsca ACOUSTIC SYSTEM (an/oar) ACOUSTIC RES/STANCE x6 OR ROOM NO. 2 1

1 I 50 ROOM No.1 1 I usm's l MOUTH & I EARS I J F76. /8/4 72 OR 76) [70 OR 74 5 82 Y 48 OR 54 L44 0/? 52 /NVE/VTOR K 5. JOHNSON BVQW ATTORNEY Dec. 26, 1950 K. s. JOHNSON 2,535,681

ELECTROACOUSTIC SYSTEM AND MEANS Original Filed Aug. 7, 1946 5 Sheets-Sheet 5 FIG. .23v FIG. 24

FIG. 25

120 NET F I6. 27 CO. OR P, 8.x. wifigglkosfb J r- 5 ACTUAL ACTUAL I L $28. 2 a ART'IFI- ART/Fl- 1 f/AL LOOP LOOP L K 3 203 'L zao' 20 202 /5/' 20/ R27 /N 5 N 7'01? K. 5. JOHNSON 8 Patented Dec. 26, 1950 I ED STATES PATENT "OFF;

si na :B T ep La oratories; ncarporated,.New YorlnN. Y.,, a corporationofNew Y rk Original application August '7, =Cl946, Serial No. 688306. "Divided land this application May 6, 1949, Serial :No. 31,762

TTClpaims. g This is a division of application Serial No. 688,906, filed August 7, 1946, now Patent No. 2 ,516,7'76, issued July/225, 1950, forElectroacoustic i stem and Means. I This inventionyrelates to electroacoustictransducer systems, and, more particularly, to conference or distant-talking systems, public address systems and telephone station circuit arrange-. ments.

. ln communication systems or circuits employs ins electroacoustic transducers, for example,

sound wave reproducers or loudspeakers, and transmittingfdevices or microphones, and particularlyin public addresssystems, and conference or two-way distant talking loudspeaking systems,

amplificationin the electrical portion of the system is generally desirable and frequently maybe essential. If this amplificationis not to result in undesirable echo orreverberant efiects, the acoustic output from the reproducer or .loudspeaker either should orimust cause a negligible effect only at the input ofthe transmitting device or microphone thatis, over the frequenflylal gfi to bearnplified; or, in the case of a two-way loud- ,speaking system...similar effects shouldbe caused orproduced at both ends of thecircuit.

An object ofihis inventionistomake available conference or distant-talking loudspeaker fiystems and devices.

Another ,objectlis ,to simplify telephone station circuits ofthe anti-.sidetone type by obviating any need for induction-type transformers therein.

.Still ..another object is to provide improved public address systems and devices.

Afeature ,oftthe invention is the use eta single electroacoustic transducer both as a sound .wave pick-up device or microphone and as a sound wave reproducing .device or loudspeaker, and associating a substantially identical transducer and appropriate network therewith, to control the acoustic dissymmetry of the room. auditorium or other space in which the public address orsound wave amplifying system isto be used.

Another feature comprises enclosing the sound wave pickup device ormicrophone, an'dthe amplifying sound wave reproducer r loudspeaker comprising the public address or sound wave amplifying system, in a symmetrical acoustic chamber or bridge, having at least a pair of acoustic openings, one opening connecting the chamber with the room, auditorium or other space in which the user or speaker is located, and the other opening leading .into or connecting the chamber with an appropriate balancing'irnpedance.

Still anotherfeature comprises the employment of .suchan enclosing arrangement, inwhich the balancing impedance is obtained by an actual physical one. simulatedduplicate of the space in which the-speaker or user of the system is located.

.A 1 further feature of the invention comprises em oy n a a ci ie wo k-tv f epeater circuit including two or more electroacoustic transducers, one or more being located ,in the space or room containing the user of thelarrangement, and one or more being locatedin a second balancing .ordummy room or acoustic chamber.

a i nal fea ur com ise u lic address or distant-talkingtloudspeaking system, in w ch ac ust c coup i g ay ex b en the sound wave pickup device and the reproducing device without causing undesirable singing.

Still another feature, involves doubly conjugate electroacoustic transducer systems. obviating the need for anyinductive transformervin an anti: sidetone telephone station circuit orof the so? calledhybrid coder-transformer type in tele-- phone repeater circuits.

Other objects and features of the inventionwill become evident from the, detailed and general d scriptio s that fo o e e a ter- A more complete understanding Of-tzhQiIlVBfle tionwill be. derived from the. detailed; description that follows, taken in; conjunction with the, appended drawings, l wherein:

Fig. '1 .-sh0W .n blockrdiagram form, .OIlBStfl-v tion of atwo-wayr distant-talking loudspeaker. or conference system which Willlbereferred to for discussing features of the. present invention;

Fig. Z is the-electricalcircuit analog of :the arrangement of Fig. 1

Fig. 3 shows, in block diagram form, a public address system whichwill'be referred to in discussing features of the present invention;

.Figs. 4. and. 5 show. electrical circuit analogs of the arrangement shownin Fig, .3;

Fig. 6 shows a blockdiagram included .ior purposes of discussion of the factors involved in electroacoustic systems ofthe type to which the presentinvention relates;

iFigfi'? illustrates an electroacoustic system, .jfor exam le. a publi ddre sy emo str cturally accordance with the invention;

Fig. 8 shows anclectrical circuitanalq to be discussed withreferfince to the arran smcn f of Fig.7;

1 10. .11 andilz il us r te n i-s de ne tele hone s et oacirc embod in a lurality .o t oe m l mpe a cc elementsi n awa d.

ance with thisinvention, to obviate use of either induction coils or hybrid coils in circuits of this type;

Figs. 13, 14, and 16 illustrate circuits corresponding to those of Figs. 9, 10, 11 and 12, but in which the impedance elements are shown specifically either as inductive impedances or capacitive impedances;

Fig. 17 illustrates a two-way repeater type transmission system in accordance with the invention;

Fig. 18 illustrates a closed acoustic system or bridge, specifically, a public address system or one station of a two-way distant-talking loudspeaker system, in accordance with the invention;

Fig. 18A shows a modification of a portion of the structure of Fig. 18;

Fig. 19 shows a public address system including a plurality of balanced electroacoustic structures and an amplifying means for the system, in accordance with this invention;

Fig. 20 shows a modification of the arrangement of Fig. 19, in which a pair of two-terminal impedance elements replaces the hybrid coil included in the arrangement of Fig. 19;

Fig. 21 and Fig. 22 illustrate the terminal circuit arrangements for two-way, distant-talking conference or loudspeaker systems embodying the principles of the arrangements of Fig. 19 and Fig. 20, respectively;

Figs. 23, 24, and 25 illustrate electroacoustic systems otherwise identical with the arrangements of Figs. 19, 20 and 21, respectively, except that series-connected pickup device or microphone and receiver or loudspeaker replace the single electroacoustic transducers of Figs. 19, 20 and 21;

Fig. 26 illustrates an electroacoustic system similar to that of Fig. 22 except that series-connected pickup device or microphone and receiver or loudspeaker replace the single electroacoustic transducers of Fig. 22, and the hybrid coil of Fig. 22 is a replaced by a plurality of two-terminal impedance elements in accordance with the principles of this invention; and

Fig. 27 illustrates a further modification of the invention as applied to a telephone transmission system.

Communicating systems of the public address, conference or distant-talking loud speaking types have received considerable study heretofore from the standpoint of minimizing, or otherwise controlling, acoustical and electrical feedback therein that would otherwise introduce undesired, or undesirable, singing, echo or reverberant effects in the system. One such system is disclosed in R. J. Tillman United States Patent No. 2,269,565, January 13, 1942, entitled Communication System, the circuit arrangement of which is illustrated in simplified schematic by Fig. 1 herein.

Itis essentially an anti-sidetone station circuit in which there are amplifying means and loss networks. A microphone M, amplifying device or means AI, and a variable loss network NWI are included in the transmitting branch, and coupled to a transmission line L through an equalizer network NW5 and a hybrid or antisidetone induction coil and balancing network It). A pair of loud speakers RI, R2 amplifying device or means A2, and a variable loss network NW2 are included in the receiving branch, and are coupled to the transmission line L through the aforesaid equalizer network and hybrid coil and balancing network. Switching means SW (including hangover relays, rectifying elements and other components) is provided to vary the loss and the gain between the microphone and the loud speakers and the hybrid coil. Fig. 2 shows an electrical circuit analogous of the arrangement of Fig. 1, the corresponding elements bearing corresponding reference designations, the network N being the analog of the circuit components matching or balancing the impedance of the transmission line.

A feature of the Tillman arrangement appears to consist either of shaping the switched loss, or of attempting to make the acoustical path between the loud speakers and the microphone effectively a conjugate one, that is, to make infinite, or as large as possible, the acoustical transfer impedances between the electrical receiving or acuostical transmitting path and the electrical transmitting or the acuostical receiving path. This acoustical impedance is to be large, especially at those frequencies at which the electrical portion of the system has a large amount of gain, so that the overall electrical and acoustical system will have a loss in it at all frequencies, and, therefore, the system will not embody undesired feedback or singing. With reference to Fig. 1 herein, this would mean that any electrical currents entering the receiving branch or the loud speakers R1, R2, shall not be responsible for producing sufiicient electromotive force in the transmitting branch, as a result of a finite acoustical transfer impedance between the receiving and the transmitting elements in the system, to cause the system to sing. 1

In the recently issued United States Patent No. 2,348,629, May 9, 1944, entitled Public Address System of the present inventor, there is shown means for obtaining good acoustical balance, together with high efiiciency of coupling between the mouth of the user of a public address system and the microphone of the latter, as well as between the loud speaker of the system and the users ears. Fig. 3 herein, corresponding to Fig. 2 of Patent No. 2,348,629, is a simplified schematic of the system of this Johnson patent. Fig. 3 illustrates a public address system including a single diaphragm (D), double converting element (T, T) microphone, a loudspeaking telephone receiver R, and an electrical amplifying means A interconnecting the microphone and receiver. In the arrangement of Fig. 3, the acoustically conjugate path was proposed to be obtained by the use of effectively two sound wave pick-up devices or microphones (actually one pick-up device), and one telephone receiving or reproducing device, combined with acoustic networks in such a way as to offset the efiects of acoustic dissymmetry in the room, auditorium or other space in which the public address system was to be used. The anti-sidetone circuit arrangement of Fig. 4 is an electrical analog of the Johnson patent arrangement. In the arrangement of Fig. 4, the components Ni, N2 are balanced against the components N and L (representing a transmis sion line), where the components N i and NZ represent the electrical equivalents of the acoustical elements of the public address system, and the components L and N are the electrical equivalents of the acoustic paths between the receiver R and the microphone. Essentially, the arrangement of Fig. i is an anti-sidetone circuit of the type shown in Fig. 5 in which a single network or component N balances the impedance of a transmission line L.

In the Tillman patent arrangement, the conjugate path is obtained by the use of two receiv aut sm;

in eelem'ents androne stransmittencelementmmmiscrophone. No meansappearsstocbe:innludedmi the Tillman arrangement for compensating for differences in the acousticaltransfer impedances between the microphone and the two receiving elements or devices.

Considering this acoustic problem from a fundamental standpoint, itis to be noted that there are four primary elements;- exclusi-ve' of variableroom condititins, involved in -th'e acousti-a- 1 a i ticaltr ansfer impedancebetween the rncutl i and Hi therear of anindividual is -presumably not under-- much r controh and approaches "a relatively a low values The "valuesof the acoustical tI EESfQP'lm pedal-ice; Z, between the mcuthl and the loudspeaker, and that; Y;'" betweenthe microphone -30 and the earsp-arenot directive?importantco nse quence; =1 The primary problem is to con'side'r what wouldbe' the =best-possih1e physicalarrangea ment 'so that the *acousti'cal transfer impedance betweenthe loudspeaker an the microphone will" i be as :near infinity-'- as possibleat all frequencies" transmitted eifl(iientlfls' by the 'ele etric'al system;

and 1 at 1% title sande -time tomake the acoustical transfer impedance between the mouth" and the microphone as well as-between' the loudspeaker 1 40 phone were identical', this would hethe equiva lento-of h'aving the componentsfiv .-of the ele'c trical analog shown imFifgrz equat, which 'case the microph'ohe --a1'i'di. i the oudspeaker wrenches would-he conjugate'r- A similar sitiiati'on would 5 hold" true the arrangements of Figs; 3 and?) ii th' "mlCIOphOIlB elements =could=be acoustically balanced with respect to=-th-receiver3 As -alrea'dyi pointed but, however; thisvbalancemustb' obtained without unnecessary sacrifice in th acoustical efllc iency between themouth and tn microphbne and between the loudspeaker and: the ear."

T6=provid-aclarer understanding ofthis; ref erenceis mad to 'the showing of- Fi Fi" showsPa hollow acoustic chamber I 2 tially' symmetricalconfieuration; for" 8XamD1}-'" spherical: or cylindric'al'f in which are positionedasz'sound wave energy -pick up deviceor "mi'c'ro-" phone -T'l, and fa -S0und""\VaiV8 energy reproducing deviceorlouds peaker R'l-Fthe'ch'amber being pro videdfwith' a pair' of acoustic "paths or-openings M63 One pathr ld prcvides "an acoustic con piih'g between the "interior :"of the -chamber antithe room, autlit'orium;- or othler-rspaceeiri 'whichthe user is *locatedj as indicated by the block des ignated 18. The other pathdfi couples the 'interidr of -the chambenwith aim-acoustic structure ontnetworkdflf simulatihg thewacoustical charace e teristics of the room or auditorium. The sound-F 6 wave translating:memberszor:diaphragmsi'eDly: DMZ of thermicrophone V and :loudspeak'er are :Larrangedii' substantially aturightr anglesyor conjugatetoronee: anotherrrlniefiectfithese translatingmemberswillil; bei acoustically :conjugate :toceach other; provided-s that there isxequality between the two racoustica impedancesr looking-into thesacoustical paths :1 and'fil 6:

The arrangement of Fig. 7 might be consideredaz at afii-st; approximationpas rthe sacoustical. equiv;- alent lof the I antr-sidetone :stationrcir'cuit arrangesmentvshownninmFla: .8; in whichrta :microphonet: or :transmitter .TB a receiver sR8,'x avtransmissin line L8; a lihe balancing :networkSNB; and .a multie r windingrinduction-coil IC are included: Theo-ea ceive'r R 8 'and .thertnansmitter; TB rwilll lbe cone jugate',=that 'ispin 'antieside'tone relation; providedzi thatltherimpedanceor the network N8 is i'approapriately proportioned fwith zrelatinn to: that of"? the impedance looklng inte -the lihe LBJl Th1 line impedance corresponds to the acoustical: im pedance looking into the acoustic path 14, and, that ofthe networkNS corresponds to the accus tical impedance looking into the acoustical path I From this latter described analog; it will be seen that the most eflicient way in' which the loudspeaker and= the microphone-of the "acousticsystem can be made conjugate isyfirst to have the-"microphone andfthe loudspeaker inherently balanced with respectto each' other in arren closed-chamber rrom which=there arenapain" acoustic: paths or openings; one 1 leadin :to-th room or "auditorium in which: the user of the ar= rangement is located," and the other-of which simulates the "impedance "of the first path 1 so that acoustic" balance is obtained in the-same ways-as the corresponding 1 electrical-balance is obtained: in? anti sicletone circuits: If 'th'e acoustic'al' imei pedances care known, for the fr'eq e s which *thesystem" is to function: the acoustical design is comparatively:simple becauseof thiselectrical analog, As already indicate'd however; this analog'islnot complete since,-' in th'earrange ment: ?of Fig? 8, an -iinductionf coil' is *included whereas, in the: 'arrang'ement of Fig: 7, there are 3 no acoustical elements having mutual impedance-: There will now be described aovnumber of doubly conjugate circuits and systems of the public address, conference' 'an'cl telephone station types; in which elements or "components "having ;substim=- tia lly' no mutual impedance betweemthem are employed.

Four of such general type of anti sidetonetel phone stationpircuits are shbwn in Figs. 9 was inclusive. In each of these circuits;six -circuit elements or components are employedf namely; a transmission or telephone line 12, a" soundwaver energyrpick up' device; that is, wtwo-termihal l microphone ortransmitterflff a sound wave "en;

ergy reproducing device; that is, a two=terminal telephone receiver or loudspeaker R; a two-terminal network N for balancing the impedance of theline; and a pair of auxiliary, two-terminal impedance elements A, Bhaving substantially no" ,3, this concept of a transformerless anti-sidetone station circuit. In each circuit, it will be noted that the balancing network and the impedance elements are connected in series across the line terminals, the microphone is connected in series with one impedance element across the line terminals, and the receiver is connected in series with a second impedance element across the line. terminals.

In Fig. 9, the microphone T is connected in se-. ries with impedance element A, and the receiver R is connected in series with impedance elements 13 across line terminals 5, 5. In Fig, 10, the receiver R is connected in series with impedance element A, and the microphone T is connected in series with the impedance element B across the line terminals 5, 5. In Fig. 11, the microphone T is connected in series with the impedance element B, and the receiver R is connected in series with the impedance element A across the line terminals 5, 5. In Fig. 12, the receiver R is connected in series with the impedance element B, and the microphone T is connected in series with the impedance element A across the line terminals 5, 5. In each of Figs. 9 to 12, the network N and impedance elements A, B are con nected in series across the line terminals 5, 5.

As already noted, the impedance elements A, B may be resistive, inductive or capacitive impedances. that of Fig. 9, with an inductance coil 1 representing impedance element A and capacitor C providing the capacitance representing impedance element B of Fig. 9;" and the circuit of Fig. 14 corresponds to that of Fig. 10, with capacitor C providing a capacitance representing the impedance element A, and the inductance coil Zrepresenting impedance element B. The circuit of Fig. 15 corresponds to that of Fig. 11, with the capacitor C providing the capacitance representing impedanc element A, and inductance coil 2 representing impedance element B; and the circuit of Fig. 16 corresponds to that of Fig. 12, with the inductance coil Z representing impedance e1ement A, and the capacitor C providing a capacitance representing impedance element B. In general, for receiver-transmitter conjugacy where ZA, Zn etc. are the impedances of A, B, etc.

A clearer understanding of the principles involved in the circuit arrangements of Figs. 9 to 16 will be derived from a consideration of the impedance relationships that should exist in a typical one or these circuits, for example, as represented in Figs. 9 and 13.

With an electromotive force acting in the microphone T of Fig.9 or Fig. 13, i. e., when a user of the circuit is transmitting out of the station circuit, there will be no current in the receiver R, i. e., no sidetone, provided Th circuit of Fig. 13 corresponds to tion circuit, there will be no current in the' balancing network N, provided or p (2) ZI ZR When Equation 2 is satisfied, the circuits-of.-

Figs, 9 and 13 reduce in efiect, on receiving-to ones in which series circuits of the microphone. and one impedance element (or an inductive reactance) and of the receiver and the otherimy pedance element (or a capacitiv reactance), are; connected in parallel across the line terminals.

If the receiver and the microphone are equal resistances, or are designed to present equal-resistances in the circuit or if their product is constant, and

gi= R T the impedance of the station circuit will be a con-v stant pure resistance, that of the receiver or oi the microphone, at all frequencies. The energy ratio, Y, that is, during receiving, the ratio of the energy delivered to the transmitter to that delivered to the receiver, is a function of the frequency as well as of the product of Z1 and 20, the function being an inverse one if the element 1 and th element C are interchanged in position.

Anti-sidetone circuits of the general type il lustrated in Figs. 9 to 16, inclusive, may beemebodied in two-way, one repeater (amplifier)-; twoway, two repeater (amplifier) and four-wire repeater (amplifier) telephone circuits, to obviate the need for hybrid-type transformers or coils-in such repeatered circuits. By way of specific example, Fig. 17 illustrates a two-way, tWOrlBPSEttQl' telephone circuit embodying a circuit of the type illustrated in Fig. 9. In Fig. 17, a line balancing network N1, and impedance elements A1, B1 are connected in series across the line terminals 6, 6 for the two-Way voice frequency telephone line. W, and a line balancing network N2 and impede. ance elements A2, B2 are connected in series across the line terminals i, l for a secondtWQ- way voice frequency telephone line E the input terminals of a voice frequency repeater or ampli fying device Am being connected across the series connection of network N1 and impedance element A1, and its output terminals being connected across the series connection of network N2 and impedance element B2; and the input terminals of a second voice frequency repeater or amplifying device A27 being connected across the series connection of network N2 and impedance element A2, and its output terminals being connecte ed across the series connection ofnetvvorkNpand impedance element B1. If amplifiers of the so: called mechanical type, that is, of thetypeincluding an acoustically coupled telephone, receiver;

element 20 and a telephone microphone element 30 are used for the repeaters A17; A21, the cost of such a repeater circuit could be kept-to a mink mum. In this connection, it may be noted from the conjugacy relations, as indicated'by Equae tions 1 and 2, that the balancing network is not, in general, equal to the line impedance unless the latter is a substantially pure resistance, but the balancing network and the line must be inverse networks of constant resistanc products. -This fact, however, does not in general complicate the design or use of the described type of-repeater circuit. .3

or, diaphragm D6 extending across a second of,

the tubular. members, for example, member 42, and exposed on each surface thereof for radiation of sound wave: energytherefrom; acoustic impedance elements A 8, NIB positioned in a pair of the tubular members, for example, mem-- ber. 40 and member 40, respectively, and extending across the passages in such members; a member or horn58 defining an acoustic path or opening 68, connecting the acoustic bridge with the room, auditorium or other space, shown by the broken-line. block. designated 62 in which the user of the system is located; and a member or horn 64, defining a second acoustic path or opening 66, connecting the acoustic bridge with asecond room or space shown by the brokenline'. block designated 68, or leadin into an acoustic impedance network simulating 0r duplieating-the acoustic characteristicsof the space 62. The coupling between the member 58 and the tubular members 44, 48 may be diaphragms I0, 12 in the walls ofthe tubular members, and the coupling between the members 64 and the tubular members 52, 54 may be diaphragms 14, 16 in the walls of'the latter mentioned tubular members. The impedance element AIB and the network NIB may be acoustic resistances of the type described in the P. B. Flanders article "Acoustic Attenuators, Bell Laboratories Record, August, 1988, page 403, et seq. The enclosed acoustic structure is symmetrical in configuration with the microphone TIB and receiver RI8 in symmetrical relation, and with impedance element AIG and network NI 8 equidistant from the microphone TIB and from the receiver R18. The members 58, 64 are symmetricall disposed with. reference to the microphone TIB and the loudspeaker RIB. The characteristic acoustic impedance of the tubular members is preferably equal to that of the diaphragms and of the acoustic impedance elements in the tubular members. The acoustic attenuation of the tubular members themselves may be negligibly small. The path or opening to the room containing the user ofthe system corresponds to the line L, the acoustic resistances AIS, NIB correspond to the impedance element A and thenetwork N, the microphone TIB and receiver RIB correspond to the microphone T and the receiver R, respectively, andthe opening or path to the room or acoustic impedance E8 and the latter correspond t the impedance element 33, of; Fig. 9. The terminals A, B of the microphone TI8 and the terminals C, D of the loudspeaker RIB are interconnected through a voice frequency amplifying means or device A28. If the user of the arrangement described with reference to. Fig. 18, is, as noted, located in the region designated 52, and the region designated 88' is'ra reasonably good acoustic or other simulation. of the region 62,, the acoustic system is obviously, by symmetry, balanced, and the loudspeaker RI8 and the microphone TIE. will be acoustically conjugate, thereby theoretically permitting any desired amount of gain inzthe electrical portion of the arrangement between the terminals A'.-B and. C -D.'. The: arrangement of Fig. 18, obviously, may be included in apublic addresssystem, or in one terminalof a conference system or a. two-way dlSt'IllllJrtfllkillg loudspeaking telephone system.

Fig. 18A illustrates an alternative arrangement for coupling regions 62, 63, to the acoustic bridge. The openings in the endsof the members 58, or 54, may be closed by a diaphragm. mechanically linked to the diaphragms, 10,12 or 1 1, '53 by appropriate linkage 82, the latter diaphragms being disposed across theadjacent endportions of tubularmembers at, 458-013 52, 54.

A somewhatsimilar system to that of Fig. 18 in that it depends upon the use of two balanced acoustical rooms, areas or spaces, orone such room, auditorium or the like and a balancing electroacoustic impedance, isshown in the arrangements illustrated by Figs. 19 to 22, inclusive, of the drawings. Fig. 19,, for example, shows an electroacoustic transducer RIii positioned in a room, auditorium crother space I00 in which the system of Fig. 19. might be used, and a second electroacoustic transducer NIB,- which may: be a substantialduplicate of the transducer Rllhlocated or positioned in a second room IIlI, which may be a substantial duplicate of the room I00, or: may beanacoustical simulation thereof. The transducers RIB, NIO areinterconnected through a three-winding transformer or hybrid coil I02 and avoice frequency amplifying means or device A10. The voice currents winding I05 of the transducer RI 0 is connected to one input terminal I03of amplifier AISand to one terminal I 0610f winding I01 of the transformer. The voice currents winding IB-B'of the transducerNIll is connected to theterminal I03 and to one terminal I09 of thewinding I I0 of the transformer. The other input terminal I04 ofthe amplifying means Al9-is common to the inner ends of the windings I01, H0. The output terminals ofthe am,- plifier AI 9 are connected to terminals III and IIZof the. third transformer winding N13. The transducers RIO, NI 0 may be of the so-calleddynamic or moving coiltype of loudspeaker, with thetransducer Rlllbeing used both as the sound wave energy pick-updevice or microphone and as the sound wave reproducing deviceor loudspeaker forthe publicaddress system; The electrical circuit arrangement bears a similarity to the ,so-called 2l-type repeater circuit used inthe telephone art, with the electroacoustical mean-s represented by theroom I0! and transducer NI 0 corresponding to the balancing network and transformer I02 corresponding to the so-called hybrid coil; In the operation of the system; sound wave energy impressed on the transducer R10: will cause voice frequency currents to be generated in the coil I05 which will traverse the winding Inland will be impressed across theiirput terminals of the amplifying means AI 9'. The latter voice frequency currents are amplified by the device A50 and impressed across the winding H3. The amplified voice currents in winding I13 will be induced in windings I01, H0, and, if their number of turns and polarity have been appropriately chosen, the division of such induced currents between the transducer Elf! and the transducer NIIl-will be such that their effectat the inputterminals I03, I04 of the amplifier vice AI 9.

ducer NIB is connected to the second output terminal II'I, of the device AIS, which may also be will be balanced out, with the currents induced in winding lil'l eifective on the transducer RIIl to cause the latter to reproduce the amplified voice currents as amplified sound wave energy directed into the room or auditorium I98.

As already indicated, the impedances Z1. and ZN looking into the electrical sides of the transducers Rlt, Nit,

are such that only a negligible amount of the electrical output of the device AI!) will be impressed across the input of such device. In this fashion, any tendency toward singing or undeof impedance element A20, which may also be one of the input terminals of amplifying device AIS, and to one output terminal N6 of the de- The voice currents coil of the transthe second terminal for the impedance element A29, and also to the second input terminal N8 of the device Al9. The impedance element B28 is connected between the terminals H6, I I8.

Fig. 21 illustrates the application of the principles of the arrangement of Fig. 19 to one station of a two-way conference or distant-talking loudspeaking telephone circuit or system. In the arrangement of Fig. 21, the transducers RI ll, NIB

are interconnected with a two-way voice frequency transmission line or loop L2I through a plurality of amplifying means or devices A2I, A22 and a balanced type hybrid coil I I9 and line balancing network I20. In the arrangement of Fig. 21, the device A2I is arranged to deliver amplified voice frequency currents outgoing to the line L2I through the hybrid coil I I9, and the device A22 is arranged to receive voice currents from the line L2I and to deliver amplified voice currents through the transformer I02 to the transducers RID, NIU of the station. As with the arrangement of Fig. 19, choice of appropriate constants for the transducer NI and the transformer I02 will minimize undesired feedback between the two one-way amplifying paths.

The circuit arrangement of Fig. 22 is the same as that of Fig. 21, but with impedance elements A22, B22 re lacing the transformer I02. in accordance with the principles discussed hereinabove with reference to Figs. 9 to 16, inclusive.

In some ca es. it ma be fo nd d s rab e to employ a separate pick-up device or microphone and a separate sound wave reproduce! or loudspeaker, in the arrangements of Fi s. 19 to 22 instead of a single device for transmi ting the sound wave energy into voice currents of corresponding frequency, and vice versa. In such event, the arrangements of Fi s. 19 through 22 may be readily modified as illustrated in Figs. 23 to 26, inclusive, the series-connected microphone TL and receiver or loudspeaker RL replacing the device RH], and the series-connected microphone TN and receiver or loudspeaker RN replacing the de vice NIB; Fig. 23 otherwise corresponding to Fig. 19, Fig. 24 otherwise corresponding to Fig. 20, and Fig. 25 otherwise corresponding to Fig. 21. The arrangement of Fig. 26 corresponds to that of Fig. 22, with the additional modification of the substitution of a pair of impedance elements A26, B26 for the hybrid coil H9, in accordance with the principles developed hereinabove with reference to Figs. 9 to 16, inclusive.

Fig. 27 shows another embodiment of the invention, being illustrated in a multistation, twoway voice frequency transmission system or circuit. The system comprises subscriber stations, SUBI, SUEZ; a central ofiice or a P. B. X station I50; and transmission paths or lines I5I, I5l', interconnecting the subscriber stations and the central office or P. B. X circuit. Each subscriber station comprises series-connected sound wave energy pick-up device or microphone T21, T21, and sound wave energ reproducing device or loudspeaker R21, R21, positioned in a semi-enclosed chamber or housing 200, 200', shown in dotted line. The transmission paths, I5I, I5I', are substantially equal electrically to each other, preferably, if necessary, being built out to equality by means of an actual or aritificial loop or network 20I, MI. The position I56 may comprise repeating coils 293, 203', a so-called hybrid coil 284, and an amplifying means or device 205. In an alread existing system that its to be modified in accordance with the arrangement illustrated by Fig. 27, the characteristic of the transmission path between the subscriber station and the central oifice could be controlled automatically by means of varistor or thermistor network at the subscriber station or in the transmission path, the characteristics of such networks being controlled by direct current (source not shown for simplicity) flowing in. the system. In the ar rangement of Fig. 27, the microphone and the loudspeaker at each subscriber station are shown in series, and as being in a partially enclosed acoustic chamber since acoustic coupling between the station instruments will not cause the overall system to sing even though a Very high order of amplification should be employed in the system. A principal requirement for maintenance of the non-singing condition is the equality of the acoustic coupling or feedback at each of the subscriber stations, so that the impedances Z1. and ZN looking out of the amplifier or repeater loop 202 do not difier appreciably at an amplified frequency.

If, in any of the embodiments of the invention referred to, the electroacoustic structures are inefficient, the amount of electrical amplification that can be used may be made much higher than it could if the structures were efficient, the assumption being that the same acoustical output may be obtained in either case, if the change in efiiciency of the electroacoustical structures is similar at all frequencies, and if an unlimited amount of amplification is available.

Although this invention has been disclosed with reference to a number of specific embodiments, it will be evident to the skilled in the art that additional modifications and applications of the principles of the invention may be made without departing from the spirit or scope thereof.

What is claimed is:

1. An electroacoustic tranducer system comprising a hollow structure containing a pair of electroacoustic transducers, one of said transducers for converting sound wave energy into electric current substantially corresponding to the sound waves and the other for converting audio frequency current into sound wave energy substantially corresponding to such audio current, each of said transducers including a diaphragm, said diaphragms being spaced apart and said structure defining substantially equal-length paths to the opposite surfaces of the diaphragm of said one transducer for sound wave energy that may be generated by said other transducer, said structure including a pair of acoustic paths, one of said paths constituting a passage to the outside of said structure for sound waves originating in said other transducer and a passage to said one transducer for sound waves that may be generated outside of the structure, the second path simulating the impedance of the first path.

2. In combination, a hollow structure containing a loudspeaker and a microphone, each of said loudspeaker and said microphone including a diaphragm, said structure defining substantially equal-length paths to the opposite surfaces of the diaphragm of said microphone for sound wave energy that may be generated by the diaphragm of said loudspeaker and including a pair of acoustic paths, one of said paths constituting a passage to the outside of said structure for sound waves that may be originated in the loudspeaker and access to the microphone for sound waves that may be originated outside of the chamber, the second path simulating the acoustic impedance of the first path.

3. The combination of claim 1 in which said chamber is symmetrical in configuration.

4. An electroacoustic transducer system comprising a hollow structure defined by a plurality of interconnected tubular members, a first electroacoustic transducer for converting sound wave energy into electric currents corresponding substantially to said sound waves and a second electroacoustic transducer for converting voice frequency electric currents into sound wave energy corresponding substantially to said currents, said structure including a first acoustic path for egress from said structure of sound wave energy that may be generated by said second transducer and for ingress into said structure to said first transducer of sound waves that may be generated outside of said structure, and a second acoustic path to the outside of said structure simulating the acoustic impedance of said first path, said transducers being positioned in different ones of said tubular members, but in symmetrical relation to said acoustic paths.

5. An electroacoustic transducer system such 14 as is claimed in claim 4 in which each of said transducers includes a sound wave translating member, the interconnected tubular members are arranged in symmetrical configuration, and an individual acoustic resistance extends across each of two of said tubular members, said acoustic resistances being located equidistant from the wave translating member of the first transducer and equidistant from the wave translating member of the second transducer.

6. An electroacoustic transducer system such as is claimed in claim 4 in which each of said transducers includes a sound wave translating member, the interconnected tubular members are arranged in symmetrical configuration, and an individual acoustic resistance extends across each of two of said tubular members, said acoustic resistances being located equidistant from the Wave translating member of the first transducer and equidistant from the wave translating member of the second transducer, said tubular members being of negligible acoustic attenuation and of characteristic acoustic impedance substantially equal to that of said acoustic resistances and the sound Wave translating members of the system.

'7. An electroacoustic transducer system such as is claimed in claim 4 including electric current amplifying means interconnecting said transducers whereby electric currents that may be generated by said second transducer are amplified and transmitted to said first transducer to be converted into amplified sound waves by said first transducer.

KENNETH S. JOHNSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Num er Name Date 2,177,769 Erickson Oct. 31, 1939 2,208,160 McCreary July 16, 1940 2,227,580 Harry et a1 Jan. 7, 1941 2,301,744 Olson Nov. 10, 1942 2,348,629 Johnson May 9, 1944.- 2,431,962 Rettinger Dec. 2, 1947 2,485,405 01ney Oct. 18, 1949 

